Commas

Serial commas

In a series of three or more items, use a comma before the final "and" or "or."

Examples

Not recommended: I dedicate this book to my parents, Ayn Rand and God.

Recommended: I dedicate this book to my parents, Ayn Rand, and God.

Commas after introductory words and phrases

In general, place a comma after an introductory word or phrase.

Examples

Recommended: Finally, only groups that contain parameters appear in this list.

Recommended: Based on the requirements of your game, you can implement this method to update game information.

Commas separating two independent clauses

When a coordinating conjunction (and, but, or, nor, for, so, or yet) separates two independent clauses, insert a comma after the first clause (before the conjunction) unless both clauses are very short.

Examples

Not recommended: The libraries not only make feed creation easier but they also ensure that only valid feeds are produced.

Recommended: The libraries not only make feed creation easier, but they also ensure that only valid feeds are produced.

Not recommended: Type your ID, and click OK.

Recommended: Type your ID and click OK.

Commas separating independent from dependent clauses

When an independent clause and a dependent clause are separated by a coordinating conjunction, insert a comma only if the sentence could be misunderstood without one.

Examples

Not recommended: Direct-access flags are plain variables, and can be read directly.

Recommended: Direct-access flags are plain variables and can be read directly.

Not recommended: The manager acknowledged the last team member who entered the room and started the meeting.

Recommended: The manager acknowledged the last team member who entered the room, and started the meeting.

Setting off other kinds of clauses

It's often a good idea to set off certain kinds of clauses with a comma or other punctuation, for clarity.

A couple of specific places where commas are a good idea:

  • In general, put a comma before the word "which" at the start of a nonrestrictive clause. For more on this topic, see this guide's section on relative pronouns, and Grammar Girl's page on which vs that.
  • In general, put a semicolon or a period or a dash before a conjunctive adverb, such as "otherwise" or "however" or "therefore," and put a comma after the conjunctive adverb.

Examples

Not recommended: Name of the group which has a maximum length of 200 characters.

Recommended: Name of the group, which has a maximum length of 200 characters.

And:

Not recommended: The variable must have a value otherwise the server returns an error.

Recommended: The variable must have a value; otherwise, the server returns an error.

Punctuating numbers

For information about punctuating numbers, see Commas and decimal points in numbers.

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Google Developer Documentation Style Guide
Google Developer Documentation Style Guide